We have not space to figure more of these creatures, but other forms will be found not inferior in interest to those mentioned. The most curious of all are those which earn a dishonest and lazy living by attaching themselves to the bodies of other and larger animals, chiefly fish. One of the largest is the _Argulus_, the bane of aquarium keepers, which is of considerable size, and attacks gold-fish, and in fact almost any fish to which it can obtain access. The gills of the stickleback will furnish examples of the curious _Ergasilus_, which consists chiefly of an enormous pair of hooks and two long egg-bags, the latter, in varying form, being carried by many of the Entomostraca. Upon the fins of the same fish will be found the remarkable _Gyrodactylus_, a worm-like animal which attaches itself by a large umbrella-like foot, in the centre of which are two huge claws. The head is split down the middle for some distance. We may mention, in concluding our notice of the external and involuntary guests of the unlucky stickleback, that its skin is usually frequented by hosts of the Trichodina described in the last chapter. Of the internal parasites, want of space forbids us to speak. CHAPTER X Marine Life--Sponges--Infusoria--Foraminifera--Radiolaria--Hydroid Zoophytes--Polyzoa--Worms--Lingual Ribbons and Gills of Mollusca--Star-Fishes and Sea-Urchins--Cuttle-Fish-- Corallines--Miscellaneous Objects. Great as is the range of objects presented to the student of fresh-water life, the latter field is limited indeed as compared with that afforded by the sea. The Infusoria and Rotifers furnished by the latter are, indeed, much fewer in number and variety, but the vast host of sponges, polyzoa, hydroids, crustacea, molluscs, ascidians, and worms, to say nothing of the wealth of vegetable life, renders the sea the happy hunting-ground of the microscopist. Whether it be along the edge of the water, as the tide retreats, especially after a gale; or in the rock-pools; or, perhaps best of all, upon those portions of the shore left uncovered only by the lowest spring-tides, the harvest is simply inexhaustible. Stones turned up will exhibit a world in miniature. Encrusted with green or pink sponges, or with gelatinous masses of ascidians, fringed at its edges with hydroids, coated above with polyzoa, a single one will often supply more work than could be got through in a week of steady application.